1. Field of the Invention
This invention relates to an oxide etch process which is highly selective to nitride. More particularly, this invention relates to an oxide etch process with high selectivity to nitride which can be used on surfaces of uneven topology.
2. Description of the Related Art
A significant challenge in semiconductor fabrication is to etch an oxide such as a silicon oxide, in the presence of a nitride, such as a silicon nitride, without also etching the nitride, i.e., while maintaining a high selectivity to the nitride. An example of such could be where an oxide layer to be etched is located over a nitride layer. Since both oxide and nitride materials generally etch at the same rate in a typical fluorocarbon chemistry etch plasma commonly used for etching oxide, a process of providing additional selectivity must be found.
When a fluorine-substituted hydrocarbon, such as C.sub.3 F.sub.8, is used as an etchant, the fluorocarbon radicals react in the plasma to form a passivating coating of a carbon-fluorine polymer which forms over the materials being etched, e.g., by exposed oxide and nitride surfaces on a substrate. However, this polymer is dissociated by oxide atoms formed during the etch of the exposed oxide portions. Thus, as the oxide, e.g., silicon oxide, continues to etch, the nitride portions etches at a much slower rate due to the presence of the passivating coating thereon. However, the passivating layer is also attacked by free fluorine atoms present in the plasma, and thus the nitride also continues to be etched. Thus, a selectivity over about 8:1 of silicon nitride to silicon oxide was not achievable with such prior art etch processes due to the presence of such free fluorine atoms in the plasma. Since for state of the art devices having submicron dimensions, i.e., VLSI and ULSI devices, selectivity of over 10:1 and even 30:1 are required, an etch process for etching oxide in preference to nitride with a selectivity of over 10:1 would be highly desirable.
In U.S. Pat. No. 5,423,945, the subject matter of which is hereby incorporated by reference, we described and claimed the provision of a scavenger for fluorine such as a source of silicon or carbon. This scavenger for fluorine, when used in combination with such fluorine-substituted hydrocarbon etch gases, results in the formation of a carbon-rich polymer which does not dissociate over nitride surfaces, apparently due to either the reduced free fluorine content in the plasma, or the reduced fluorine content in the polymer, or both. In any event, this use of a scavenger for fluorine in combination with fluorine-substituted hydrocarbon etch gases results in an oxide etch having a selectivity to nitride, i.e., a preference for oxide etching over nitride etching, of over 10:1, and up to as high as approaching infinity.
However, more recently an additional problem has been discovered where at least the nitride surfaces of the nitride/oxide structure being etched are not flat, as for example, the sidewalls of a slot or on raised steps such as, for example, nitride-coated polysilicon lines. This type of structure is illustrated in FIG. 1 wherein raised polysilicon lines 10 and 12, formed over a substrate 2, are coated with a conformal layer 20 of nitride, over which is formed an oxide layer 30 and a photoresist mask 40.
When oxide layer 30 is etched, through mask opening 42 in photoresist mask 40, down to conformal nitride layer 20, nitride portions 22 on the sidewalls of raised polysilicon lines 10 and 12 are also at least partially etched, indicating that the above-described protective polymer is either not forming on such generally vertical surfaces (surfaces generally perpendicular to, or at least not planar with, the underlying substrate), or the protective polymer is being more readily attacked by the etchant gases on such nonplanar surfaces than the corresponding polymer portions formed on horizontal surfaces, i.e., surfaces generally planar to the underlying substrate, such as nitride portion 26 between raised lines 10 and 12.
It would, therefore, be desirable to provide an oxide etch process highly selective to nitride which is suitable for use on surfaces of uneven topography wherein nitride surfaces which are not planar to the underlying substrate, including sidewalls on slots or raised portions, will be resistant to etching by the oxide etch process.